Marx is a young man with fair hair in a bowl cut hairstyle. He has narrow eyes with long lashes and a typically serious expression on his face.
He wears a dark, long sleeved formal jacket under a light, furred mantle with an attached robe that reaches nearly to his ankles. The front of his mantle is decorated with an ornate chain and clasp. The collar of his jacket is trimmed with fur and has six diamond-shaped ornaments placed horizontally across the front.
Dutiful and serious, Marx is often frustrated by the Magic Emperor's tendency to shirk his duties and go off searching for new magic. He frequently loses his composure when dealing with Julius's starry-eyed outbursts about various magic, and struggles to keep the Emperor on task.
Though a high ranked mage who works alongside Clover's top officials, Marx has an amiable and courteous personality as shown when he treats Asta with respect despite his low status.
After Julius Novachrono undoes his Transformation Magic spell, Marx contacts the Magic Emperor and tries to talk to him about the emergence of a dungeon, but Julius keeps interrupting and, to Marx's surprise, is already aware of the dungeon and has someone in mind to handle it.
As the Magic Emperor is away during the attack on the Clover Kingdom, Augustus Kira Clover XIII is concerned that Julius is not there to protect him, but Marx assures the king that he and the other advisers of the Magic Emperor will protect him. Although, on the inside, he is also annoyed and wonders where the Magic Emperor went.
As soon as the Magic Emperor opens his Communication Magic spell, Marx contacts him. He is surprised that the Magic Emperor knows about the attack already and how it went. He asks the Magic Emperor to come to the medical ward, as Fuegoleon Vermillion's situation is not good.
After helping Julius address the citizens of the Royal Capital, Marx tries to extract information from Catherine and George, the captured members of the Eye of the Midnight Sun. Unfortunately, his attempts are blocked by a protect magic spell on their minds. Julius asks his aide to keep investigating while they wait for Fuegoleon to open his eyes.
Weeks later, Marx greets Asta when the boy wakes up after the battle in Nairn. Marx asks for his help with the prisoners and leads him down to the dungeon, where they meet up with Julius. The Magic Emperor bombards Asta with questions about the magic he saw, and Marx shouts to refocus Julius. After Asta removes the protection magic on George and Catherine, Marx begins their interrogation anew. They reveal that Gueldre Poizot is the traitor, so Marx calls the captains down to the dungeon so they can hear for themselves. Gueldre denies the accusation and questions Marx's magic, but Marx swears by his magic. After Gueldre tries to escape and is captured by Rill Boismortier, Marx begins interrogating the Purple Orca, revealing the captain's shady history and his part in enabling the invasion. Julius dismisses the other captains and has Marx continue the interrogation.
A week later at the Royal Knights Selection Exam, Marx once again accompanies Julius. When Zora Ideale arrives and puts his arm around the Magic Emperor, Marx threatens the rogue, who is forced to join the other Magic Knights.
Several days later, Marx sees Julius fighting Patolli and alerts the Magic Knights squads. After Patolli fatally stabs Julius, Yami orders Marx to fetch Owen, and as Marx flies back with Owen to the tower, they are possessed by elf spirits and attack Magic Knights along the way. When they threaten to harm Julius's body, Yami goes on the offensive and quickly knocks them unconscious.
A few days later, Marx rushes to Julius's room and is flabbergasted that he is still alive and has turned into a child. Marx begins crying in relief so Julius hugs him and apologizes. Marx refuses to forgive him so easily and informs him about the mounting pile of paperwork.
Days later, Marx receives a call from Mimosa Vermillion and the Princess of the Heart Kingdom, Lolopechka. Julius interrupts and pushes Marx aside, and Marx scolds him for letting others see his weakened state. After some discussion with the Princess, Julius agrees to let the Knights help the Heart Kingdom. He also suggests going there himself, which Marx refuses to allow.
- Memory Magic: Marx uses this magic attribute to manipulate memories. Combined with his Communication Magic, Marx can probe and display the memories of individuals in order to get information. According to Marx, the information obtained using this magic is absolute.
- Communication Magic: Marx uses this form of magic to transmit and project auditory and visual information.
- Magic Bullet: Marx is capable of manifesting and concentrating his mana on his hand. Subsequently, he can launch the mana towards an opponent, blasting them away.
- Marx ranked #40 in the first popularity poll, #44 in the second, and #53 in the third.
- Marx's grimoire design is the background for Order of Knights Book's cover.
- Marx's favorite things are tea between jobs and, generally, Julius.
- The elven tattoos on Marx's cheeks are similar in design to the USB trident symbol, a reference to his Memory Magic.